Ask Electronics

Yes, you'd damage the car's electrical system. First of all it's not designed to feed in energy through that outlet. It's made to output energy.

And most importantly: 24V is way too much. 2 times the intended voltage would fry most electronics. Your stereo, the power steering, airbags, ... There is a good margin and car electronics are designed to be pretty robust, but you're pushing it.

I think they're still fine because what happens is your car battery absorbs that extra voltage. But it's really dangerous. On a sunny day you'll charge your car battery beyond the 14V or so the chemistry can handle. And at that point it'll degrade fast. The acid in there is going to start to boil, producing hydrogen, so in addition to a destroyed battery, you're in for a small explosion if you're very unlucky. And once the battery is gone it'll start frying the cars electronics because now there isn't anything keeping the voltage down.

Get a switch that exclusively connects either the car or the solar panel to the bluetti. One switch that switches between two things, not an On/Off switch. And make sure it's rated for the current.

Edit: Or a relais that toggles between both. It can switch if there's power on the 12V rail, and connect the bluetti to either or.

What the everloving fuck

This is a bad idea.

Beyond the hostage imbalance, you really need a storage system between the solar panel and device you want to use. Without it, power supply can be inconsistent.

A car output is going to be ~12-14Vdc and the solar panel at 24V. You do not want to connect those two together. Ideally you have some kind of switch that connects the input to the bluetti only to the solar panel or only to the car outlet, and to the car output only if it is present. You can probably use a contactor or a heavy duty relay that is 12V and connect the NC contact to the panels, NO to the car, and COM to the bluetti. Add a capacitor to dampen spikes whenever the system switches and connect the grounds together. A bjt jank, and also have to check the relay ratings / coil power usage

I'm pretty sure this is the same thing as crossing the streams; charge reversal of the proton and the ensuing complete destruction of the universe. Your car's electrical system should be fine, though it might be the only thing left in all of existence.

Put a 12V to 24V inverter just inside the firewall. This will isolate the "noisy" vehicle power system from the more fragile solar panel and power bank. It won't be nearly as expensive as an inverter that can survive the engine bay.

The 24V output of the inverter should be wired in parallel with one or several 24V solar panels, then into the power bank input. In this configuration the vehicle power system and solar system can charge the power bank simultaneously. No action is needed to change between power modes. However, you should have a main cutoff switch, disconnecting all power from the power bank.

I highly recommend a Victron brand inverter.

Warning: Direct current (DC) systems present an extreme fire danger. Every wire must be the correct gauge, every fuse the right size, every crimp correct. No piece of the puzzle is difficult. But, one must have all of them to build a safe and reliable system.

The solar panel needs to be used with a charge controller. Connecting it directly to a battery will overcharge and damage the battery. Charge controllers typically don't like the battery being disconnected while the solar panel is still connected, so don't connect it to a switched outlet.

Sounds like a bad idea unless the power input can handle 2 different voltages into the same place (at once?).

If it can handle both at once, you could still damage the car by feeding 24v into a 12v output port. It might blow a fuse, or burn up a circuit, or do nothing.

But it might be possible to install a diode or diodes in the line to prevent power flowing back into the car. Diodes are like one-way gates for electricity, but it would need to be the right kind and rated for the volts/amps going through it

Connecting them in parallel means the solar panel's 24V output gets mixed with the car's 12V systems. It's not uncommon for car systems to run a little high like 14-16V to charge the battery but 24V is definitely quite on the high side. That might make the lead acid battery explode and will probably damage other systems as well.

This might be fixable with diodes but I feel like that's probably still not ideal to try to mix 12V with 24V on the same circuit. I would put a switch and have it be like a toggle kind of deal. Or maybe some connector that toggles a switch on insertion similar to headphone jacks that disables the speakers when plugged in.